That's urban slang for pimpin', stunnin', or lookin' fly made popular by the rap group 3 six mafia. Or if someone wants their respect or recognition for doing something. Kind of like when the Fonze on 'Happy Days' use to turn his collar up to be cool.

Poppin' My Collar is the best song by Three 6 Mafia, and one of the better rap songs to come out in a while. The remix with Project Pat is the hottest version of the song. If you don't have this song you better go out right now and buy it.